The present invention relates to a novel rare earth/iron/boron-based permanent magnet alloy composition capable of giving permanent magnets having greatly improved magnetic properties.
As is well known, the demand for rare earth-based permanent magnets is rapidly growing in recent years by virtue of their very excellent magnetic properties enabling a compact design of electric and electronic instruments with a permanent magnet built therein as well as the relative economical merits obtained by using such a high-performance permanent magnet. In order to further increase these advantages, it is now desired to upgrade the magnetic properties of rare earth-based permanent magnets more and more. Among various types of rare earth-based permanents magnets, rare earth/iron/boron-based magnets, referred to as the R/Fe/B-based magnets hereinafter, or, in particular, neodymium/iron/boron-based magnets are highlighted as compared with earlier developed samarium/cobalt-based magnets in respect of lower material costs because of the much greater abundance in nature of neodymium as the principal rare earth element constituting than samarium and saving of expensive cobalt in addition to the much better magnetic properties than samarium/cobalt-based magnets.
A great variety of proposals and attempts have been made heretofore with an object to improve the magnetic properties of the R/Fe/B-based permanent magnets. For example, Japanese Patent Kokai 59-64733 and 59-132104 teach a R/Fe/B-based magnet alloy composition with admixture of titanium, nickel, bismuth, vanadium and others as additive elements with an object to obtain a stabilized coercive force of the magnets. Japanese Patent Kokai 1-219143 proposes addition of 0.02 to 0.5 atomic % of copper to the R/Fe/B-based magnet alloy so as to improve the magnetic properties of the magnets along with an advantage in the productivity of the magnet products due to broadened tolerance for the range of temperatures in which the magnet body is to be subjected to a heat treatment. Further, Japanese Patent Kokai 1-219143 reports an increase in the corrosion resistance of R/Fe/B-based magnets by the addition of 0.2 to 0.5 atomic % of chromium to the magnet alloy.
Starting from the above mentioned copper-admixed R/Fe/B-based magnet alloys, the inventors conducted extensive investigations for further improving the magnetic properties of the magnets by the addition of various kinds of other additive elements. The attempts in this direction, however, were not very fruitful because the improvement in the coercive force of the magnet obtained by the addition of most of other additive elements is accompanied by a decrease in the residual magnetic flux density practically to cancel the improvement in the coercive force.